Ship project history: An enlargement and an
improvement of the 'A' class, these boats, also designed by Eng Vice Admiral
Edgardo Ferrati, were provided with two different kind of motors, having a
petrol motor for surface navigation and a greater endurance. Their torpedoes
were in real tubes. They too had been conceived for harbour defence and were
transportable by railway. B4-6 construction suspended in 1917, discarded
1919, scrapped 1920.